1. Field of the invention
The present invention relates generally to a conveying device for a sheet type object such as paper sheets, thin film strips, magnetic tape, etc. More specifically, the invention relates to paper sheet feeding devices using piezoelectric or ultrasonic element as a source of a driving force. The device of the present invention can be used particularly effectively in various office equipment and business machines such as copiers, printers, facsimile, etc., especially when small size and weight are required for compactness.
2. Description of the Prior Art
Paper sheets, thin film, magnetic and other tape, and other similar thin and low weight objects, generally referred to as a "sheet" object throughout this description, are used extensively in various office equipment such as copiers, printers, video- and audio- tape recorders and players, photo- and video-cameras, etc. Conventional sheet feeding and conveying devices contained in these machines are typically based on a feed roller driven by an electrical stepper motor coupled with a gear box. Although widely used, these devices limit the opportunities for making the business machines more compact due to the complexity of such arrangement as well as the inherent size and weight of the stepper motor and associated gear box and sophisticated control electronics. Further efforts to make business machines small and light demand the use of a more simple but yet reliable sheet feeding device.
Piezoelectric and ultrasonic motors and paper sheet conveying devices are also generally known in the prior art. Occasionally, they are used to drive the feeding rollers of the device instead of a stepper motor as suggested for example in the U.S. Pat. No. 5,548,176 by Oda. Piezoelectric motors and sheet feeders offer an advantageous balance of small size, simple control electronics, good energy efficiency, and enough power for transporting such light objects as a sheet of paper. Therefore, their use in business machines as paper feeders has gained acceptance in recent years.
Two broad categories of piezoelectric conveying devices can be identified analyzing the prior art:
devices with a "surface contact" in which the area between the piezoelectric element and the base support structure is wide so that it is the surface of the vibrating element itself that provides a firm contact with the conveying sheet and determines the direction of transporting, and PA1 devices with a "linear contact" in which the area between the piezoelectric element and the base roller has a small width of about 0.5 mm or less so that the rotating roller in turn feeds the sheet in a predetermined direction.
Surface contact piezoelectric sheet feeders are quite reliable in operation, typically utilize a "traveling wave" principle and contain at least one piezoelectric disk divided into appropriate sectors. In a typical document feeder, a sheet of paper is placed between the flat portion of the vibrator and a support base which can also be a piezoelectric disk by itself. The traveling wave vibrations generated in the piezoelectric disk transmit a directional force down to the paper sheet and consequently move it in a specified direction.
An example of such a design can be found in the U.S. Pat. No. 5,348,287 by Yamamoto. A pair of "running track" type vibration members are arranged in a vertical direction to form traveling waves so as to face each other. A paper sheet is placed between the vibration elements and is moved by frictional forces generated by the interaction between the vibration elements and the paper sheet itself. A paper sheet with even thickness is propelled in a predetermined direction. One of the disadvantages of this type of sheet feeders becomes apparent when a sheet of slightly uneven thickness enters the device. Due to this asymmetrical thickness, an oblique and partially sideways movement of the sheet is produced.
Another example is disclosed in the U.S. Pat. No. 5,244,202 by Nishimoto and describes a sheet feeding apparatus including a vibration member with at least one circular-arc portion for generating a traveling wave oscillations. This invention attempts to cure the uneven sheet thickness problem by providing a sheet guide preventing a sideways movement of the paper sheet.
Additional examples of this type of a sheet conveying device can be found in U.S. Pat. No. 4,672,256 by Okuno; U.S. Pat. No. 4,736,129 by Endo; U.S. Pat. No. 4,955,598 by Hiroshige; U.S. Pat. Nos. 5,062,622 and 5,065,999 by Kataoka; U.S. Pat. Nos. 5,085,423 and 5,499,808 by Nishimoto; U.S. Pat. No. 5,094,444 by Seki; U.S. Pat. Nos. 5,176,376 and 5,211,390 by Igaki; U.S. Pat. Nos. 5,348,287 and 5,642,949 by Yamamoto.
All these devices have intrinsically limited ability to accurately transport a thin sheet of paper or another similar material due to an uneven thickness, stiffness and other irregularities usually encountered in a conveying sheet. Such disturbances lead to increased power consumption, changing the feeding direction, possible scratching or other damage to the conveying sheet which in turn led to creation of various compensation devices designed to correct these problems. Overall, a practical balance between an ultra-small size and weight of the device and a feeder accuracy and low energy consumption is very difficult to achieve.
Linear contact sheet feeders resolve the problem of uneven thickness of the feeding sheet by switching from a surface contact to a line contact with the feeding sheet. Most known devices of this type contain a "standing wave" vibrational element in which the working edge of a piezoelectric member moves in a cyclical pattern and is placed in direct contact with the conveying sheet supported on the other side by a spring-biased roller or another support surface.
An example of such device is found in U.S. Pat. No. 3,747,921 by Knappe in which the document feeding device contains an electromechanical transducer transforming energy to an elastomer coupler element by producing strain waves. The coupler element transmits strain waves to the document. The oscillatory motion of the contact surface of the coupler element in cooperation with a confronting support surface defmes a document path in a selected direction.
Another example of a linear contact piezoelectric sheet feeder is found in the U.S. Pat. No. 4,997,177 by Mori. Here, a vibrating-type driving unit employing a piezoelectric device is provided for conveying a sheet object placed between a follower roller and a piezoelectric device. By selecting a friction coefficient of the roller to be greater than the friction coefficient of the driving head, a stable sheet transporting function is achieved.
Further devices of this type are proposed in U.S. Pat. No. 4,999,536 by Toda; U.S. Pat. No. 5,071,113 by Nakamura; U.S. Pat. No. 132,582 by Hayashi; and U.S. Pat. No. 5,233,258 by Myoga.
Generally speaking, linear contact conveying devices have several important advantages over the surface contact feeders: they are quite simple, more compact, have lower weight, highly energy efficient, very accurate, have favorable "start/stop" parameters, and fairly inexpensive to build.
One of the limiting factors associated with conveying devices of this type arise from the fact that the vibrating element is placed in direct contact with the conveying sheet. High reliability of operation can only be achieved with firm contact between the vibration element and the supporting roller. However, placing a sheet of paper between these two elements can cause damage to the sheet and therefore demands reducing of the contact force. The need exists therefore for an piezoelectric device which eliminates this limitation.
Linear contact piezoelectric motors are also known in the prior art. These devices contain a vibration element placed in direct contact with the turning roller. However, one traditional drawback associated with these otherwise attractive devices is their limited longevity due to the failure of the contact zone elements between the vibrating element and the roller surface after a relatively low number of working hours, leading in turn to a loss of the rotational torque and a general failure of the device. A typical example of a piezoelectric motor of this type is described in the U.S. Pat. No. 4,019,073 by Vishnevsky et al.
The need exists therefore for an improved low power sheet conveying device of the linear contact type with extended operational life. Such device would allow the utilization of all intrinsic advantages of this type of devices and therefore allow for significant reduction in the weight and the size of business machines and other general devices requiring a sheet feeding function.